Jim Michalak's Boat Designs

118 E Randall, Lebanon, IL 62254

A page of boat designs and essays.

(15February 2012) This issue will take on the chine runner issue with an essay about underwater board shape. The 1 March issue will continue the topic.



is out now, written by me and edited by Garth Battista of Breakaway Books. You might find it at your bookstore. If not check it out at the....


...which can now be found at Duckworks Magazine. You order with a shopping cart set up and pay with credit cards or by Paypal. Then Duckworks sends me an email about the order and then I send the plans right from me to you.


Wojtech Baginski sails his Polepunt on the river at Warsaw, Poland, last year.



Contact info:


Jim Michalak
118 E Randall,
Lebanon, IL 62254

Send $1 for info on 20 boats.




Sizing Underwater Boards

This issue will tell you how to figure the size for the "underwater board" that is used to balance the side loads produced by the sail.

By "underwater board" I mean a leeboard or daggerboard or centerboard or fixed fin or keel. I don't see how the exact type makes much difference as far a abililty to counteract side load goes. There are other practical differences of course.

As in previous discussions I'll assume the underwater board will counteract all the lateral force of the sail. "But," you might say, "the immersed hull itself provides some lateral resistance." True, but that is usually quite small in proportion to that of a good fin because of aspect ratio considerations that we'll go over in the next issue. You might also ask, "Hey, if you make all the sail's force side force, what's to drive the boat forward?" Correct again, but when close hauled a sail produces mostly lateral force with only a small of forward force left to push the boat forward. It's a vector thing and vectors don't add up like regular numbers. For example if you 100 pounds of sail force directed 60 degrees off centerline, you would have 50 pounds of force pushing the boat forward and 86 pounds pushing to the side. Assuming all of the sail's force is side force doesn't introduce a huge error.

I'm going to use Frolic2 as an example again. Remember in previous issues we worked up a righting moment curve for the boat using the Hullforms6S program we got as freeware at Blue Peter Marine's web page. From there we found that the maximum righting moment for the boat with two big men sitting to windward was 1550 ft pounds. The sail (114 square feet) and leeboard areas are 12 feet apart on Frolic2 so the maximum sail force we can stand without capsize is 129 pounds (which happens in about 15 knots of wind).

The force on the leeboard is assumed to be the same as the lateral force of the sail at that time, so the maximum force on the board would be 129 pounds. If it exceeds that the boat will capsize with that crew configuration. How do we size the leeboard such that we can be pretty sure it will produce that 129 pounds of lateral force?


The leeboard, or any underwater board, "flies" through the water in the same way that an airplane wing flies through the air. If the board is pointed dead ahead into the flow of water, it produces no lateral force, only drag. If a small "angle of attack" is introduced, a large amount of lateral force can be produced. To produce that angle of attack , skipper need only point the boat slightly upwind of his desired course. So the boat points one way and goes slightly downwind of where it is pointed. You might view this as "leeway".

The equation for the lift, or lateral force, developed by the board is F=2.86 x S x C x V x V. It's a lot like the equation used for figuring sail force in the wind. In fact it's exactly the same equation with an allowance that the density of water is about 900 times the desity of air. But there are some other differences.

The S in this equation is the area of the underwater board in square feet. Remember it is only the area that is immersed in the water flow.

In the case of the underwater board, the value of C is, I would think, less that the C of 1.5 that I recommended for sails. The reason is that the underwater boards cannot have camber, as a soft sail can have, and function through tacking left and right. It must be symmetrical. (OK, boats with two leeboards can have cambered boards.) Thin airfoils with no camber seldom have maximum lift coeffecients much greater than 1. So I recommend the value used for C in this equation be 1. Then the equation reduces to F=2.86 x S x V x V.

(Below is shown a chart of lift coefficients for various fins of various "aspect ratios". I'll explain what aspect ratio is next issue. This chart was gleaned from info from Marchaj. Looks like his test data indicated the C maximum was 1.2 but the chart seems highly idealized, which is OK. Besides the note of C max, note that the general trend is for each fin to gain C directly proportional to the angle of attack until the maximum is reached. Then it levels off.)

V in this case is the boat's speed through the water, not the wind speed. How fast is your boat going to go? Boy, is that a tough question. The speed you want to use in the equation in not the "hull speed", the usual assumed maximum speed of the boat The hull speed (in knots) of a displacement boat is often shown as about 1.3 times the square root of the waterline length (in feet). So the Frolic2, with a waterline length of about 18' would have expect to have a top speed of about 5.5 knots. But the speed we want to use in the equation is about half of that maximum speed. Why? Because for the underwater board, the worst condition is when beating to windward at low speed. At that time the load on the sail, and thus the load on the leeboard, is maximum, but the boat's speed is well below maximum as it beats through the waves. Let's call this something like "beating speed". So for Frolic2 we might take the beating speed to be 2.75 knots. (It's still pretty fast for a sailboat going to windward.) So now for Frolic2 the equation becomes F=2.86 x S x 2.75 x 2.75. which is equal to F = S x 21.6. If we think the maximum force on the board is going to be 129 pounds as per our stability analysis, we can solve for S = 129/21.6 =6 square feet of leeboard area.

(Here is a chart that you can use to figure the pressure that water exerts on an underwater board at various boat speeds. I've shown two values of the force coefficient, C=1 that I recommend, and C=1.5 for you optomists.)

That is actually a pretty large board for Frolic2. As designed it has about 4.4 square feet of leeboard immersed. What does that mean? That means that with two big boys sailing on the rail in 15 knot winds, they need to maintain a speed that will produce about 129/4.4 = 29 psf on the leeboard. If you look at figure 2, you will see that is 3.2 knots. If they don't do that here is what happens. The crew will slide off to leeward. Rudder movements intended to increase the board's angle of attack and thus its lift will do no good. About the only things they can do to regain complete control would be to head downwind a bit (to increase the boat's speed), or let out the sheet a bit to feather the sail somewhat and thus reduce the sail load to something the board can handle. If they try to "pinch" the boat closer to windward, their hull speed will diminish more, they will be stuck, bouncing up and down in the waves, probably going one foot sideways for each foot forward. I've been there many times! How about you? (Another solution is to reef to reduce the sail force.)

Now if you don't like doing math, I'll tell you where I probably got the value of 4.4 square feet when I designed Frolic2. It turns out that if you have a pretty normal boat and make the board are 4% of the sail area, you will probably have something that works quite well. Frolic2 has 114 square feet of sail, so 114 x .04 = 4.6 square feet. I guess that's how I figured it.

I do think it is possible to make the fin too big. Certainly you can add a lot of drag by making it huge. The reason I say this is that when I made my Bolger Jinni a long time ago, I used a leeboard that was about 5 feet long. The next season I chopped about 12" off its bottom. Easier to handle. And I can't say I ever really noticed any difference in its performance going to windward. As a contrast to that, I enlarged the leeboard on my Piccup Pram after a couple of seasons and noticed an enjoyable improvement in its ability to sail to windward, especially with the larger sails had I started to use.

One interesting thing about the way the board behaves is that if you have two boats with the same rig in the same wind, and one is faster than the other, the faster boat can get by with a smaller board. Why? Because its faster speed through the water produces more pressure on the board than the slow boat. But I don't think you can say something like, "I'm going to increase sail area. That will increase my speed and then I can reduce my board area." It might happen that way if you are lucky. But when you increased the sail area you increased the sail force. And you will need increased board force to balance it. Whether the boat speed increase will sufficicently increase the board force is hard to say.


We'll see that not all board areas are created equal. Deep skinny ones can get to a certain value of C a lot sooner (at less angle of attack) than shallow fat ones.




IMB features a "Birdwatcher" cabin, full length with panoramic windows and a center walkway slot in the roof. Everyone rides inside. This style of boat was invented by Phil Bolger in the early 1980's.

These boats can be self righting with minimal, or no, ballast because crew weight works as ballast. They sit low looking out through the windows (although standing in normal winds is quite acceptable). The cabin sides provide lots of buoyancy up high to ensure a good range of stability. IMB, which is small with a light bottom, should reliably self right from 60 or 70 degrees and in the test described above self righted from a full 90 degrees of roll.

These boats are operated from within the cabin, like an automobile. No one need ever go on deck. For boating with children I can see no equal.

These are usually cool inside. The tinted windows cut the sun's power. The crew can sit in the shade of the deck. Downdraft from the sail cascades through the walkway. (By the way, at the Conroe messabout two boaters with Lexan windows noted that mosquito spray will ruin Lexan with one application and they noted belatedly that the back of the spray can says so.)

IMB has an 8' long cabin on a multichine pram hull. The prototype was built to perfection by Gerry Scott of Cleveland, Texas. At the Conroe (Houston) messabout I got a chance to look over his boat plus the only other IMB I know of built by Bob Williams. Both boats were quite true to the plans. Both had added low inside seats which made them more pleasant to use to the point that I will show some seats on the plans. I was worried when I drew IMB that the headroom would be minimal so drew no seats thinking the crew would sit on the floor, as with the original Birdwatcher.

While I was sailing with Gerry, Bob's boat came out on the lake with four adult males and no sign of bogging down, showing that these fat pram shapes, very much like my Piccup Pram, can handle a lot of weight in the 13.5' length.

(Later they rescued a mermaid and returned to the dock with five total.)

I don't know if either boat had ever been weighed and the 350 pounds I quote as the empty weight is just a guess. One of the ideas behind the boat was that it might be towed behind a compact car and I was glad to see that Gerry tows his behind a 1500cc mini SUV.

Both men adjusted well to the lug sail/leeboard rig. Gerry's has the blueprint 104 square foot sail and Bob's uses the 114 square foot Bolger Windsprint sail available from Payson. I used to worry a bit about running a leeboard on a full cabin boat like this since handling must be done by remote control, so to speak. No problem. Both boats have the leeboard lanyard running to a cleat on the aft deck. The leeboard position is plainly in view at all times through the cabin window. In use these leeboards need only lanyards to pull them down. Once down they will usually stay down until they strike something. Then they pop up and you will need to pull them down again. I've never seen a need for a lanyard to pull the board up although I've seen several rigged that way. The Dovekie design had elaborate cam operated levers in the cabin that operated the leeboards and I thought that all very clever. But in talking to some Dovekie owners I found the internal levers are not universally loved since they can often be in the way. Anyway, my idea was not to run the down lanyard to the aft deck but rather through a small hole in the side of the boat, say 1/2" for a 1/4" lanyard, so it could be operated totally from inside the cabin.

Both Gerry's and Bob's boats used electric trolling motors. The plans show rowing ports and no provisions for a motor. A boat like this won't be a fast row boat but it might be useful in a calm. Even the 24' Birdwatcher would row about 2.5mph in a calm. But I'll admit that adding a motor to Birdwatcher makes it a much more useful thing.

IMB takes two sheets of 1/2" plywood, eight sheets of 1/4" plywood and one sheet of 3/16" Plexiglass. Taped seam construction using no jigs or lofting.

IMB plans are $30.


Prototype News

Some of you may know that in addition to the one buck catalog which now contains 20 "done" boats, I offer another catalog of 20 unbuilt prototypes. The buck catalog has on its last page a list and brief description of the boats currently in the Catalog of Prototypes. That catalog also contains some articles that I wrote for Messing About In Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3. The both together amount to 50 pages for $4, an offer you may have seen in Woodenboat ads. Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currency that I can admire but not spend.) I'm way too small for credit cards.

I think David Hahn's Out West Picara is the winner of the Picara race. Shown here on its first sail except there was no wind. Hopefully more later. (Not sure if a polytarp sail is suitable for a boat this heavy.

Here is a Musicbox2 out West.

This is Ted Arkey's Jukebox2 down in Sydney. Shown with the "ketchooner" rig, featuring his own polytarp sails, that is shown on the plans. Should have a sailing report soon.

And the Vole in New York is Garth Battista's of www.breakawaybooks.com, printer of my book and Max's old outboard book and many other fine sports books. Beautiful job! Garth is using a small lug rig for sail, not the sharpie sprit sail shown on the plans, so I will continue to carry the design as a prototype boat. But he has used it extensively on his Bahamas trip towed behind his Cormorant. Sort of like having a compact car towed behind an RV.

And a Deansbox seen in Texas:

The prototype Twister gets a test sail with three grown men, a big dog and and big motor with its lower unit down. Hmmmmm.....

And the first D'arcy Bryn is ready for taping. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....

And the first Brucesboat is in the water for testing. A full report soon.

OK, so he found a major league goof in my plans on fitting the bilge panels. He did some cut and fit and did a great job of salvaging the work, but I have corrected the drawing for the aft end of the bilge panel (I drew it in upside down!!)

And a Hapscut goes together in Texas. He has scarfed some material on the stern to finish the boat with a built in motor well like Laguna. Good idea:





Mother of All Boat Links

Cheap Pages

Duckworks Magazine

The Boatbuilding Community

Kilburn's Power Skiff

Bruce Builds Roar

Dave Carnell

Rich builds AF2

JB Builds AF4

JB Builds Sportdory

Hullforms Download (archived copy)

Puddle Duck Website

Brian builds Roar2 (archived copy)

Herb builds AF3 (archived copy)

Herb builds RB42 (archived copy)

Barry Builds Toto

Table of Contents